1. Field of the Invention
The present invention relates to a booster power supply circuit, a control method therefor and a driver IC, and particularly to a booster power supply circuit having a standby mode, a control method therefor and a driver IC.
2. Description of Related Art
In semiconductor integrated circuit technology, power supply voltage and power consumption have been attempted to be reduced. In a driver IC for driving LCD (Liquid Crystal Display) or the like, the power supply voltage has been reduced. On the other hand, a voltage required to drive LCD or the like is determined in advance according to pixel material, and a high voltage capable of outputting high brightness is often required so as to improve display quality. Therefore, the driver IC includes a built-in booster and a voltage for driving LCD is supplied from the booster.
There are many ICs including a LCD driver IC supplied with an external power supply for generating an internal voltage by a booster. An output voltage generated by the booster is usually connected with a smoothing capacitor to attempt to stable an output.
A driver IC having such booster built-in stops the operation of the booster while not needed to display in order to reduce power consumption. As described in the foregoing, the state not supplying a driving voltage to LCD and retaining to be low power consumption state is referred to as a standby mode. In the driver IC, to stop the operation of the booster while inputting an external power supply, which means to enter the standby mode, charge in a smoothing capacitor is discharged.
As a method to discharge the charge in the smoothing capacitor, there are for example; a method to simply discharge the charge accumulated in the smoothing capacitor to ground as disclosed by Edogawa in Japanese Unexamined Utility Model Publication No. 5-55737, a method to discharge by connecting to an external power supply, which is an input power supply of the booster, and discharging from the external power supply as disclosed by Tatsumi in Japanese Unexamined Patent Application Publication No. 6-225546 and by Morishita et al. in Japanese Unexamined Patent Application Publication No. 7-44134.
Circuit diagrams of a booster power supply circuit 10 having almost same configuration as the configuration of a booster power supply circuit disclosed by Edogawa are shown in FIGS. 17 and 18. FIG. 17 shows the state of the booster power supply circuit 10 in a normal operation mode while FIG. 18 shows the state of the booster power supply circuit 10 in the standby mode. As shown in FIG. 17, the booster power supply circuit 10 includes a booster 11 and controller 12 or the like. A first smoothing capacitor C1 for smoothing an external power supply VCC is provided to an input side of the booster 11. Further, a second smoothing capacitor C2 for smoothing a boosted voltage Vout is provided to an output side of the booster 11. Additionally, the second smoothing capacitor C2 is connected to a resistance R1 via a first switching device SW1. The resistance R1 is a resistance for limiting current.
The booster power supply circuit 10 includes a controller 12. The controller 12 outputs a STBYB signal for switching between the standby mode and operation mode. The booster power supply circuit 10 becomes the operation mode when the STBYB signal is “H” and the first switching device SW1 becomes OFF state as in FIG. 17. At this time, the booster 11 performs a normal boosting operation. On the other hand, the booster power supply circuit 10 becomes the standby mode when the STBYB signal is “L” and the first switching device SW1 becomes ON state as in FIG. 18. At this time, the booster 11 stops the boosting operation.
FIG. 19 is a view showing the configuration of a conventional booster power supply circuit. In the operation mode, the first switching device SW1 is OFF state and the second smoothing capacitor C2 functions as a capacity to stable a voltage of the boosted voltage Vout. On the other hand in the standby mode, the booster 11 stops the boosting operation with the first switching device SW1 being ON state, charge in the second smoothing capacitor C2 is discharged to ground.
As described in the foregoing, in the booster power supply circuit 10 disclosed by Edogawa, all the charge in the second smoothing capacitor C2 is discharged to ground in the standby mode. It is true that keep applying a DC potential to LCD panel is a problem in the lifetime of the panel and discharging to ground is effective. However, to discharge the charge in the second smoothing capacitor C2 when using the booster power supply circuit 10 for a power supply or the like inside the driver IC, a large amount of charge must be supplied in a transition from the standby mode to operation mode, thereby increasing power consumption.
To overcome this problem, a booster power supply circuit 20 disclosed by Tatsumi or Morishita et al. is suggested. Circuit diagrams of a booster power supply circuit 20 having almost same configuration as the configuration of a booster power supply circuit disclosed by Tatsumi or Morishita et al. are shown in FIGS. 20 and 21. FIG. 20 shows the state of the booster power supply circuit 20 when power is turned on and in the standby mode. Further, FIG. 21 shows the state of the booster power supply circuit 20 in a normal operation mode. As shown in FIG. 20, the booster power supply circuit 20 includes a booster 21 and a controller 22 or the like. A first smoothing capacitor C1 for smoothing an external power supply VCC is provided to an input side of the booster 22. Further, a second smoothing capacitor C2 for smoothing a boosted voltage Vout is provided to an output side of the booster 21. A first switching device SW1 is provided between the booster 21 and second smoothing capacitor C2. Further, the first smoothing capacitor C1 and second smoothing capacitor C2 are connected via a second switching device SW2.
Further, the booster power supply circuit 20 includes a controller 22. The controller 22 outputs a STBYB signal for switching between the standby mode and operation mode. The booster power supply circuit 20 becomes the standby mode when the STBYB signal is “L”. At this time as shown in FIG. 20, the first switching device SW1 becomes OFF state, second switching device SW2 becomes ON state and the booster 21 stops the boosting operation. On the other hand, when the STBYB signal is “H”, the booster power supply circuit 20 becomes the operation mode. At this time as shown in FIG. 21, the first switching device SW1 becomes ON state, the second switching device SW2 becomes OFF state and the booster 21 carries out the boosting operation.
FIG. 22 shows an operation output waveform of the booster power supply circuit 20. In the operation mode (where STBYB=“H”), the first switching device SW1 is ON state, second switching device SW2 is OFF and the second smoothing capacitor C2 functions as a capacity to stable a voltage of the boosted voltage Vout. On the other hand in the standby mode (where STBYB=“L”), the booster 21 stops the boosting operation, the first switching device SW1 is OFF and second switching device SW2 is ON state so as to discharge the charge in the second smoothing capacitor C2 to the external power supply VCC.
In the booster power supply circuit 20 disclosed by Tatsumi or Morishita et al., a path is provided for connecting to an input terminal from the second smoothing capacitor C2 towards the external power supply VCC. In the standby mode, the charge in the second smoothing capacitor C2 is discharged to the external power supply VCC. Therefore in the standby mode, the charge is charged to a potential of the external power supply VCC in the second smoothing capacitor C2. Accordingly when changing again from the standby mode to the operation mode, the booster 21 boosts from the potential of the external power supply VCC to the output potential Vout. Thus the wasteful transfer of the charge can be reduced to the minimum and also the time till the completion of the boosting operation can be reduced.
However in a case the external power supply VCC does not have enough capability to discharge the charge in the second smoothing capacitor C2, the potential of the external power supply VCC increases as shown in FIG. 22. Therefore, it has now been discovered that a voltage more than defined value is applied to the external power supply VCC and input part of the booster 21.
Thus, a booster power supply circuit having a high reliability that suppresses power consumption with the minimum transfer of the charge and prevents a high voltage exceeding a withstand pressure of the input part of the booster from being applied even when absorbing capability of an input of the booster is small.